The steroidogenic activity of Y-1 cells and their response to ACTH will be examined to investigate the manner in which ACTH stimulates steroid synthesis in the tumor cells. Three questions will be approached: (1) The nature and source of mitochondrial protein synthesized under the influence of ACTH will be sought by determining whether these proteins are synthesized on ribosomes associated with mitochondria. (2) Anti-actin will be used to explore the possible role of actin in the response of Y-1 cells to ACTH. (3) Mitochondrial proteins, associated on gel filtration with cholesterol, will be separated to identify those proteins which might serve as cholesterol-binding proteins and to determine whether one or more of these proteins is (are) synthesized at an accelerated rate under the influence of ACTH.